How to Determine Speeds and Feeds for Milling
(machine, materials, and tools)
Speeds
Milling cutting rates vary according to the machinability of the material being cut; whether
cutting fluid is used and, if so, what kind; the type, size, and material of the cutter and
the coarseness of its teeth; and the amount of metal being removed. Cutting speed for
milling is the distance the cutting edge of a tooth travels in one minute. If cutting speed
is too high, the cutter overheats and becomes dull. If it's too low, production is inefficient
and rough.
There is no exact right cutting speed for milling a particular material. Machinist usually
start with an average speed, then increase or decrease it as needed. For light cuts, use
the upper end. Use the lower end for heavy cuts and when you don't use cutting fluid.
Determining rpm. To set the spindle speed, you have to know the cutter rpm (revolutions
per minute). For inch measurements, use this formula:
rpm = 12 x CS (fpm) / D” x !
where:
CS
- cutting speed
fpm
- feet per minute
D”
- diameter of the cutter in inches
!
- 3.14 You can use an rpm chart for selected diameters of cutting tools at
different cutting speeds.
For metric measurement, use this formula:
rpm = CS (mpm) x 1000 / D (mm) x !
where:
CS
- cutting speed
mpm
- meters per minute
D (mm)
- diameter of the cutter in millimeters
!
- 3.14. You can use an rpm chart for selected diameters of cutting tools at
different cutting speeds.
Change Speeds by selecting the belt location and turning the speed dial
Feeds
Set the direction of feed before you begin milling. Up milling, or conventional milling, is
when the direction of feed is opposite to the direction of cutter rotation. Down milling, or
climb milling, is when the direction of feed is the same as the direction of cutter rotation.
Appendix A: Machining Reference Guide
A-5
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